This invention relates to a heat resistant expansion member in, for example, a sheet form preferable as a holding member of a ceramic honeycomb monolithic catalyst comprising a catalyst converter in a low pollution engine capable of purifying the emission by oxidizing or reducing the harmful components discharged from an automotive engine such as carbon monoxide, hydrocarbon and nitrogen oxides.
As the catalyst for achieving a low pollution engine by purifying the emission by oxidizing or reducing the harmful components discharged from an automotive engine such as carbon monoxide, hydrocarbon and nitrogen oxides, the ceramic honeycomb monolithic catalyst excellent in high temperature characteristics is preferred.
Since the ceramics are brittle and inferior in toughness, they are installed in a metallic casing being protected with a winding of cushioning holding members so as not to be damaged by the mechanical impacts such as vibrations occurring while the vehicle is running.
The ceramic honeycomb monolithic catalyst is exposed to high temperature emission of the engine, and the holding member is required to have an excellent heat resistance so as not to be lowered in the high temperature strength. What is more, since the emission is gradually heightened in temperature as the engine runs continuously, the holding member is thermally expanded depending on the temperature increase, and even in such circumstances it is required that the holding power and cushioning property for the ceramic honeycomb monolithic catalyst may not be lowered.
As an example of a holding member of the monolithic catalyst capable of satisfying such requirements, the heat resistant expansion sheet disclosed in the Japanese Laid-open Patent 56-92155 (1) and the heat resistant expansion sheet disclosed in the Japanese Laid-open Patent 58-208164 (2) are known.
The heat resistant expansion sheet (1) is composed of 40 to 65 wt. % of treated vermiculite, 25 to 50 wt. % of ceramic fibers, 5 to 15 wt. % of organic elastomer latex polymer and inorganic bonding material, and the heat resistant expansion sheet (2) is composed of 5 to 20 wt. % of sepiolite, 5 to 22 wt. % of treated vermiculite, 5 to 15 wt % of organic bonding material such as ester acrylate.
In these heat resistant expansion sheets (1) and (2), however, as clear from Table 1 herein the basis of the experimental results achieved by the present inventor, a relatively large negative expansion occurs due to creep phenomenon around 300.degree. C. corresponding to the low temperature region, thereby resulting in looseness, which is known to extremely lower the holding power of the ceramic honeycomb monolithic catalyst.
Accordingly, in order to increase the holding power of the ceramic honeycomb monolithic catalyst, an experiment was conducted by increasing the tightening surface pressure from the value in Table 1. As a result, as shown in Table 2, in spite of a relatively large negative expansion, the same as in Table 1, occurring around 300.degree. C. corresponding to the low temperature region, the thermal expansion was inhibited around 750.degree. C. corresponding to the high temperature region owing to the large setting of the tightening surface pressure, and the coefficient of thermal expansion was notably smaller than in Table 1, and it was found that the holding power of the ceramic honeycomb monolithic catalyst in the high temperature region was extremely lowered. At the same time, as clear from Table 3, it is known that the drop of the residual holding force is significant when loaded repeatedly with vibration.